Added LinuxMain.swift so numerics can be used on Linux

[tachoknight]

Wanting to use this library on Linux, running swift run --repl fails because LinuxMain.swift is not present under the Tests directory. Adding this file allows the library to work on Linux.

[stephentyrone]

swift test --enable-test-discovery, which should become the default eventually

[tachoknight]

Hm, when I try that I get **error: no tests found; create a target in the 'Tests' directory**
Creating an empty LinuxMain.swift file prevents the error, but I figured to actually include the individual tests for full coverage.

[stephentyrone]

Hm, when I try that I get error: no tests found; create a target in the 'Tests' directory

@aciidb0mb3r is that expected?

[SusanDoggie]

exec the following on Mac

swift test --generate-linuxmain

[ole]

It looks like swift build fails on Linux when LinuxMain.swift is missing unless you specify swift build --enable-test-discovery. See this forum thread: https://forums.swift.org/t/make-test-discovery-on-by-default/30321.

The contents of LinuxMain.swift are not important. It can be empty or contain a fatalError pointing people to --enable-test-discovery, but the file must exist for now.

[stephentyrone]

Hi all--as a follow-on, I've replaced this LinuxMain.swift with a file containing only a fatalError( ) with a message directing users to --enable-test-discovery. Can someone please confirm that this works as intended on Linux, and then I'll cut a 0.0.1 tag?

[ole]

@stephentyrone I tried running the tests from macOS in a Ubuntu 18.04/Swift 5.1 Docker container. The LinuxMain thing worked for fixing swift build, but there was another issue when building the tests. I sent PR #74 for that.

When #74 is merged, the tests run correctly.

By the way, if you install Docker for Mac, you can run open a Linux terminal with Swift installed by running this command:

docker run --rm --privileged --interactive --tty --volume $(pwd):/src --workdir "/src" swift:latest

This will map your current working directory to /src in the Docker container. Then you can use swift build and swift test to test things on Linux (you may have to make a clean build because SwiftPM can get confused by having build artifacts from both macOS and Linux).